1. Field of the Invention
The invention relates to a shadow mask used for a color cathode ray tube (CRT), and more particularly to a shadow mask being formed with a plurality of slots between which bridge portions are formed, which shadow mask is capable of having enhanced strength without deterioration of its brightness characteristics.
2. Description of the Related Art
FIG. 1 illustrates a widely used color cathode ray tube (hereinafter, a cathode ray tube is referred to simply as "CRT"). The illustrated color CRT includes a tube 1a having a face panel 1 at its front and a necked portion 1b at its rear, a fluorescent screen 2 formed on an inner surface of the face panel 1a and comprising stripe-shaped fluorescent films for blue (B), green (G) and red (R), and black matrix films, a mask frame 3 supported in the tube 1 in facing relation to the face panel 1a, a shadow mask 14 formed with a plurality of slots 15, an electron gun 8 disposed in necked portion 1b of the tube 1, and a deflection yoke 10 disposed outside around the necked portion 1b.
The electron gun 8 emits three electron beams 9 (only one of them is illustrated in FIG. 1), which are deflected by a horizontal magnetic field and a vertical magnetic field both generated by the deflection yoke 10 to thereby scan the fluorescent screen 2 with the thus deflected electron beams 9 through the shadow mask 14 having a plurality of the slots 15. The electron beams 9 impinge upon the strip-shaped fluorescent films for associated colors to thereby excite the fluorescent screen and cause the fluorescent screen to emit light. As a result, a certain color image is constituted on the fluorescent screen 2.
FIG. 2 is a partially enlarged partial view of the shadow mask 14, and FIG. 3 includes a partial enlarged plan view of a line of slots and a cross-sectional view thereof as viewed in a direction indicated with the line III--III. As illustrated in FIG. 2, the shadow mask 14 has a plurality of the rectangular slots 15 arranged in lines. The slots 15 in each line are vertically arranged, and are spaced away from adjacent ones by a pitch Pv, for instance, in the range of 0.2 mm to 1.0 mm. The slot lines are horizontally arranged in parallel, and between the slot lines are formed connections 17 having a constant width. Thus, the slot lines are equally, horizontally spaced away from one another.
Between the slots 15 in each line are formed bridge portions 16. In other words, the slots 15 are partitioned by the bridge portions 16. Each of the bridge portions 16 has a cross-section as illustrated in FIG. 3. Specifically, a cross-sectional area at of the bridge portion 16 is defined with a first area 16a at a first surface 14a of the shadow mask 14, increases in a thickness-wise direction of the shadow mask 14 up to a maximum cross-sectional area at 16c, and decreases in said thickness-wise direction down to a second area at 16b at a second surface 14b of the shadow mask 14. As illustrated in FIG. 3, the cross-section of the bridge portion 16 increases in an inwardly arcuate manner from the first area 16a to the maximum area 16c, and decreases in an inwardly arcuate manner from the maximum area 16c to the second area 16b. Herein, the first surface 14a of the shadow mask means a surface facing the fluorescent screen 2, and the second surface 14b of the shadow mask 14 means a surface facing the electron gun 8.
The mechanical strength of the shadow mask is defined by the bridge portions 16 formed vertically between the slots 15 and the connections 17 formed between the slot lines. The shadow mask 14 is readily broken by a bending force acting to horizontally bend the shadow mark 14. Hence, it is quite important for the bridge portions 16 to have sufficient mechanical strength.
The shadow mask 14 is in general made from a thin steel plate made of invar material, and the slots 15 are formed by forming resist patterns defining rectangular openings therein on the first and second surfaces 14a and 14b of the shadow mask 14, and etching the shadow mask 14 both at the first and second surfaces 14a and 14b. The rectangles defined by the resist patterns have a longer side in a vertical direction V and a shorter side in a horizontal direction H. The slots 15 have a common length L in the vertical direction V, and are arranged so that the bridge portions 16 formed between the slots 15 in a certain slot line are disposed at the vertical center of the slots 15 in the adjacent slot line.
Since the bridge portions 16 do not allow the electron beams 9 to pass therethrough, the bridge portions 16 form shadows on the fluorescent screen 2. The thus formed shadows deteriorate the brightness characteristic of the shadow mask 14. In addition, with electron beams 9 that are designed to have a smaller spot diameter, an even greater difference in brightness on the fluorescent screen 2 occurs with the result that images constituted on the fluorescent screen 2 are significantly influenced by Moire fringes.
If the pitch Pv between the adjacent slots 15 were kept constant, while making the length L of the slots 15 greater to thereby cause the slots 15 to have a greater opening area, the shadow mask 14 could have a greater ratio of the opening area to the entire area thereof to thereby ensure brighter images on the fluorescent screen 2. That is, the brightness characteristic of the shadow mask 14 would be enhanced. However, since the pitch Pv would be kept constant, it would be unavoidable for a maximum width Bw of the bridge portions 16 to be made smaller, resulting in deterioration in the mechanical strength of the shadow mask 14 against a bending force acting to horizontally bend the shadow mask 14.
On the other hand, if the maximum width Bw of the bridge portions 16 were made greater, it would be unavoidable for the length L of the slots 15 to be made smaller. This would enhance the mechanical strength of the shadow mask 14, but at the same time would deteriorate the brightness characteristic of the shadow mask 14.